os FS 70 overhaul question - RC Groups
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Aug 26, 2007, 04:27 AM
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Mojo_72's Avatar

os FS 70 overhaul question

Hi Guys,

I intend to change some parts for my 4 stroke 70 size to regain back the good old days power. When i took apart the engine , i saw the piston was still looking good meaning not much of scratches.

Would it be OK if i were to just change the bearings, liner and the ring only.

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Aug 26, 2007, 05:25 AM
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TomCrump's Avatar
That's what was done to mine, only just a ring, no liner. It ran beautifully.
Aug 26, 2007, 10:18 AM
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Yes the ring is the first thing, Scuff up the cylinder wall a little bit with some emery paper. Check the bearings, they may not need to be replaced. But if they are corroded, pitted, sound grundgy, feel gritty when spinning them, then they probably need to be replaced. Good idea to do it when you have the piston out anyway.

Check your gaskets upon reassebly. Make sure all the 0-rings and gaskets are in good condition.

And you will have a new engine, practically. Take some time and slow cook block and head in Anti-Freeze and clean up the out side of the engine to and it will really look like new.


Aug 26, 2007, 11:13 AM
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Mojo_72's Avatar
wow... the ring was the answer then.

Ok..will knock out those bearing as well. Thks guys
Aug 26, 2007, 01:21 PM
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Timco's Avatar
Mojo 72, just curious, what's the condition of the camshaft?
Aug 27, 2007, 11:50 AM
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Mojo_72's Avatar
Overall it is still looking good except the flywheel has turn black and rusty. With abit of polishing should do the job, i think.

Sorry for the pic quality as i am using the phone camera.
Aug 28, 2007, 08:41 PM
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That is the crankshaft not the cam shaft. The cam shaft is up on top and has a gear that meshes with the gears on the crankshaft. The push rods go down into the cam shaft housing. The cam needs to come out and be cleaned so you can see the timing mark on the side of the gear. With the engine at TDC, the dot on the gear should be on top and line up with the push rod housings, slightly off center to the rear. Timco was asking about the cam because some folks have had the coating on the cam to peel off.


Sep 06, 2007, 10:50 PM

os rebuild

probably best not to use emery cloth to scuff the cylinder. best to use 00 steel wool
Sep 07, 2007, 12:35 AM
Suspended Account
Don't do any thing to the cylinder!!! The crosshatch marks you see are in the base material not the nickel coating. OS does not perform any machining on the cylinder after it has been electroless nickel-plated. The nickel is only 0.0003 to 0.0005 inches thick. Any abrasive WILL damage the cylinder. The propose of the crosshatch is to hold oil. It is the oil film strength that actually holds the compression. Why did you think the ring was bad? The usual cause for poor low speed (hand turning) compression is the seal between the piston and ring. The ring will bed into the lower part of the piston groove. As the piston is soft aluminum it will show a lot of wear over time and as a function of over heating.

If you think you must break some kind of glaze off the cylinder use a plastic abrasive such as Scotch Brite. Please don't use a real abrasive.

Friends don't let friends fly nickel (this is one of the many reasons),
Sep 07, 2007, 10:02 AM
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ivanc's Avatar
Originally Posted by Konrad
Don't do any thing to the cylinder!!! The crosshatch marks you see are in the base material not the nickel coating. OS does not perform any machining on the cylinder after it has been electroless nickel-plated. The nickel is only 0.0003 to 0.0005 inches thick.
. . .
Konrad, are you sure OS nickel plates the liner of their ringed engines?

Sep 07, 2007, 10:29 AM
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If it is not a coated cylinder and the original crosshatch is worn, a hone for wheel cylinders works great. You can pick one up at your local auto parts store. They will fit in engines about 40 and bigger. I use lots of oil while doing this and get great results with a little practice you can get approximately the same angle as the original crosshatch. I forget the recommendation (something like 30 degrees to each other) but you can eyeball it and get close enough. Of course if it is a coated cylinder it should be cleaned and left alone.
Sep 07, 2007, 08:32 PM
Suspended Account
Yes OS nickel plates the liner and a lot of other stuff. The cross hatch for oil retention is known as 30į by 60į pattern. If you real want to hone a (not this) cylinder I would do as I do and find an automotive machine shop, one that repairs import car parts. These engines often have what is called a free-floating wrist pin. The machine shop would have a ridged hone such as a Sunnen hone. A ridged hone can make the cylinder round and take out any taper that may be in the sleeve. If you tell the machinist what the sleeve is for he might laugh and do the work for free, if he has a standard arbor. Unless you are willing to straighten out the sleeve donít bother with the honing of a crosshatch. If the sleeve is not straight and true the ring will bounce along the cylinder never really making a seal. I canít stress enough that it is the seal between the lower piston groove the ring and cylinders that separates the dog engines from the good engines.

Nickel engine donít afford any rework. Heck chrome is usually 10 times thicker than nickel and is machinable. This gives you (the machinist) a chance to add more life to an engine with a proper rebuild.

In truth I have rarely seen were the OS 4 cycle ringed cylinder needs to be reworked. I would replace the piston and ring if you think the ring was over-heated and that it has lost its temper.

Friends donít let friends fly nickel.

P.S. I hate flex hones and glaze breakers. I feel do it right or go home! This do it right philosophy is why I always say, "Friends don't let friends fly nickel"!
Last edited by Konrad; Sep 07, 2007 at 08:59 PM.
Sep 07, 2007, 08:41 PM
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Timco's Avatar
Here's a quote from the manager of Hobby Services:

"O.S. Max ringed engine s have ALWAYS had a steel liner. No plating involved."


Bill Baxter, Manager Hobby Services/Futaba Service/North America
3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
Service Phone: 217 398-0007
Email: hobbyservices (at) hobbico.com

I'm pretty sure I can dig around and find the same statement by him concerning Surpasses.
Sep 07, 2007, 08:49 PM
Suspended Account
The old two cycle engines did not have nickel plating. They were made of a higher-grade alloy. The later OS junk was made with softer alloy and the use of nickel added a good wear surface (not as good as chrome) and protected the sleeve from corrosion. With all due respect managers know little of their product unless they are the engineering manager. Most managersí job is sales and great pains does a very good job at sales.

Friends don't let friends fly nickel,

Edit: I may need to temper my comments about the use of a lower grade alloy as I never did a sampling between the 2 cycle verses the 4 cycle cylinders.
Last edited by Konrad; Sep 07, 2007 at 09:21 PM.
Sep 07, 2007, 10:59 PM
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Timco's Avatar
Here's what the late Bill Robison has to say concerning O.S. liners: "All the four stroke engines, except the FL-70, are ringed pistons running in a plain steel liner." If you will check the tech notes at the Tower web site you will find this statement corroborated.

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